1. Field of the Invention
The invention relates in general to a circuit protection apparatus. More particularly, to a protection apparatus applied to a telephone line.
2. Description of the Related Art
As the science technology becomes more and more advanced, the mutual relationship between each individual becomes more and more frequent and relative. Telephones, fax machines, modems and other appliances for telephones are necessary for daily life. Under the circumstances such as erroneously plugging a telephone connector into an integrated service digital network (ISDN) socket, malfunction of the private branch exchanger (PBX) and telephone line failure of the telephone office would cause an abnormal high voltage or current. Any appliance connected to the telephone line is very likely to be damaged by the abnormal high voltage or current. Referring to FIG. 1, a conventional protection device for a telephone line is shown. The telephone line is connected from a socket 40. The connection status of the telephone line is determined by a signal sent from a fax/modem controller 12 to hook switch 14. The protection device is implemented by connecting a fuse or poly switch 10 to the telephone line in series. When an abnormal over voltage or over current occurs, the fuse or poly switch 10 is blown out. A new fuse or poly switch has to be installed to replace the blown one to retrieve a normal operation of the fax/modem machine. It is thus very inconvenient to practical application. In European standard, a current limit circuit 20 has to be included in the circuit of the modem/fax machine. As a consequence, while an over current occurs, a current flowing though is restricted within a certain magnitude to cause a blowout of a transistor set 30 due to an over voltage, high temperature, or even high temperature of the enclosure. The protection performance is thus very limited.